a. Field of the Invention
The present invention relates to a crank case ventilation system for an internal combustion engine that has a two-stage air charge boosting system.
b. Related Art
During the normal operation of an internal combustion engine, combustion gases are forced past the piston compression rings by the combustion pressure into the crank case region of the engine. This flow of xe2x80x9cblow byxe2x80x9d gas is most noticeable on turbocharged or diesel engines because of the higher cylinder pressures typical with such engines. It is necessary to release these gases from the crank case in order to prevent a build up of pressure which could lead to ineffective oil sealing. Typically, it is desirable to maintain a crank case pressure close to atmospheric pressure. For example, one specification requires that the crank case pressure is maintained with xe2x88x922.5 kPa to +5 kPa of atmospheric pressure.
As the blow by gases contain numerous combustion products and engine oil picked up from the crank case, it is not possible to vent the gases to the atmosphere and it is normal to implement a closed Crank Case Ventilation (CCV) system that separates the oil vapour from the gases and then feeds these back into the engine intake. In the case of a turbocharged engine one way of doing this is to feed the blow by gases back into the air inlet system upstream of the turbocharger. This avoids the need for a pump in the return path of the crank case gasses that would otherwise be needed to overcome the increased pressure in the air inlet path downstream of the turbocharger.
Normally, the amount of air supplied to a turbocharged engine is measured using a Mass Air Flow (MAF) sensor upstream of the turbocharger. One important consideration is that the entry point for the crank case gasses must be sufficiently far away from the Mass Air Flow (MAF) sensor in order that any air flow reversal does not carry the blow by gases back to the MAF sensor and contaminate it.
The present invention concerns the case where two compressors are connected in series. This may be desirable, for example, if the downstream compressor is a turbocharger that becomes operational about a certain minimum engine speed, and the upstream compressor is an electrically driven supercharger that can be activated upon demand in engine operating regions where the turbocharger is ineffective.
The space in the engine compartment of a typical motor vehicle is cramped, and so the provision of two compressors makes it more difficult to package the entire assembly so that recirculated crank case gasses do not contaminate any MAF sensor placed upstream of the compressors.
It is also the case that certain types of compressor bearings can be adversely affected over time by contamination from the blow by gasses. While it may always be possible to design bearings that are less affected by such contamination, this can increase the cost of a compressor.
It is an object of the present invention to provide a crank case ventilation system that addresses these issues.
According to the invention, there is provided a crank case ventilation system for an internal combustion engine, comprising an air inlet system for supplying the engine with air, an engine air charge pressure boosting system for boosting the pressure of said air supply, and a crank case evacuation system for drawing crank case gasses from a crank case and introducing said gasses into the air inlet system, wherein:
a) the air charge boosting system has in series a first boost stage and a second boost stage, the first boost stage being upstream of the second boost stage;
b) the crank case evacuation system includes a first conduit and a second conduit for introducing the crank case gasses into the air inlet system, the first conduit introducing said gasses upstream of the first boost stage, and the second conduit introducing said gasses downstream of the first boost stage and upstream of the second boost stage; and
c) the crank case evacuation system includes flow control means for controlling the flow of crank case gasses in the first conduit and in the second conduit.
The system will also normally include an oil separator for reducing the amount of oil in the crank case gasses.
It is particularly advantageous if the flow control means is arranged to direct the flow of crank case gasses to the first conduit when the first boost stage is boosting the pressure of the air supply, and to the second conduit when the first boost stage is not boosting the pressure of the air supply. This provides a number of benefits. First, in systems where the first boost stage is operated less than the second boost stage this can then help to minimises the volume of crank case gasses passing through the first boost stage. Second, when the first boost stage is operational, the pressure downstream of this stage will in general be increased. Thus, it is not necessary for the crank case ventilation system to include any means for assisting the flow of crank case gasses in the first conduit or in the second conduit, as the gasses can then be introduced only upstream of the first boost stage.
In a preferred embodiment of the invention, the flow control means is a control valve. Both the first conduit and the second conduit can then lead from the control valve to the air inlet system. The first conduit joins the air inlet system at a first location upstream of the first boost stage, and the second conduit joins the air inlet system at a second location between the first boost stage and the second boost stage.
Also in the preferred embodiment of the invention, the crank case evacuation system includes a third conduit, said third conduit leading from the crank case to the control valve.
The control valve may be an automatic valve that is actuated automatically to admit crank case gasses to the air inlet system through the first conduit or the second conduit as required.
Preferably, the control valve is passively actuated by pressure differences within said conduits. This provides the advantage of not having to provide an electronic control system or electro-mechanical actuators to activate the valve.
The control valve may also be a pressure regulated control valve, that is, a valve actuated by pressure differences across the valve that result in the valve varying the proportion of crank case gasses delivered to the first conduit and the second conduit.
The invention also provides an internal combustion engine comprising a crank case ventilation system, said ventilation system being according to the invention.
Also according to the invention, there is provided, a method of ventilating a crank case of an internal combustion engine, the engine comprising a crank case, an air inlet system, an engine air charge pressure boosting system including a first boost stage and a second boost stage, and a crank case evacuation system linked to both the crank case and air inlet system, the crank case evacuation system including a flow control means, the method comprising the steps of:
i) using the air inlet system to supply air to the engine;
ii) boosting the pressure of said air supply using the first boost stage and the second boost stage;
iii) using the crank case evacuation system to draw crank case gasses from the crank case and introduce said gasses into the air inlet system; and
iv) using the flow control means to control the introduction of said gasses into the air inlet system at a plurality of locations in the air inlet system, including a first location upstream of the first boost stage, and a second location downstream of the first boost stage and upstream of the second boost stage.
The flow control means may then be arranged to direct the flow of crank case gasses to the first location when the first boost stage is boosting the pressure of the air supply, and to the second location when the first boost stage is not boosting the pressure of the air supply.